It is desirable that semiconductor devices, for example field effect transistors for power amplification, be miniaturized so that such semiconductor devices may be manufactured at lower costs.
However, a distance between wirings, more specifically the distance between the wirings which intersect with each other with an insulation film interposed between the wirings, may be narrowed due to the miniaturization of the semiconductor device. Such narrowing between the wirings causes a larger parasitic capacitance between the wirings and makes a high frequency operation difficult in the semiconductor device.
An air-bridge wiring has been effectively used to reduce the parasitic capacitance between wirings intersecting with each other with the insulation film interposed between the wirings. Especially, the air-bridge wirings are effective in achieving high speed operations of the field effect transistors for power amplification in which a number of field effect transistors (FETs), called a “multi-gate transistor,” are provided over the same semiconductor substrate.
FETs are arranged in one line, and terminals (sources, drains, and gates) are coupled to other corresponding terminals by comb-shaped electrodes to form the multi-gate transistor. In such a multi-gate transistor, intersections are inevitably formed at locations where source electrodes are coupled with a plurality of sources and gate electrodes are coupled with a plurality of gates.
To avoid contact of a gate electrode and a source electrode, a bridge wiring structure is provided at the intersection. The bridge wiring structure is a structure where one electrode crosses over the other electrode with an overhead crossing with an insulation film being interposed between both electrodes (For example, see Japanese Laid-Open Patent Application 2003-197740.).
The parasitic capacitance becomes greater in the bridge wiring structure due to a high dielectric constant of the insulation film provided between the crossing wirings. In consequence, air-bridge wirings where the crossing wirings are separated by a space are used in achieving the high-speed operations of the multi-gate transistor.
Gold (Au) having a high electric conductivity is widely used as a metal layer that forms the air-bridge wiring. However, gold is a soft metal and maintaining the air-bridge structure is difficult for gold alone. Consequently, a laminated structure that includes titanium (Ti) and platinum (Pt) is used as a support body to form a Ti/Pt/Au laminated structure, and the air-bridge wiring is formed. Here, Ti is used to achieve better adhesion between the substrate and the air-bridge layer, and the Au layer is supported by a Pt layer (For example, see Japanese Laid-Open Patent Application 2007-150282.).
In recent years, high electron mobility transistors (HEMT) in which channel layers are formed of gallium nitride (GaN) have been attracting much attention as a high frequency and high output transistors (Hereinafter, a HEMT whose channel layer is formed of gallium nitride (GaN) is referred to as a “GaN-HEMT”.).
Since the band gap of GaN is wider in comparison with those of Si and GaAs, the GaN-HEMT is suitable for operations under high temperatures. Moreover, since the breakdown voltage of the GaN-HEMT is high, the GaN-HEMT is suitable for operations at high voltages. Consequently, GaN-HEMTs may have fewer malfunctions due to an increase in operation temperature or an increase in electrical field, even when the GaN-HEMTs are miniaturized and operated under a large current.
For this reason, multi-gate transistors which include GaN-HEMTs are used as high frequency/high output power amplifiers.